A soft-start switch is a switching device placed between a power supply and a load. The soft-start switch when first turned ON provides to the load a voltage that gradually rises from zero to some desired level. Often the rise in voltage takes the form of the familiar rising voltage vs. time curve of a charging capacitor in an RC circuit. See, for example, FIG. 1 where the voltage supplied to the load, denoted as V.sub.out, exponentially rises to a reference voltage, denoted as V.sub.ref.
It is desirable to add a current limiting feature to a soft-start switch so that the current supplied to a load is kept below some maximum current value, so as to prevent excessive current damage to the load and the connectors, and to reduce unwanted perturbations in other circuits powered by the power supply powering the soft-switch. For example, a hard-disk drive when first powered-up is largely a capacitive load, and if it is powered-up by a simple switch it is possible that an excessively large current may be drawn by the hard-disk drive.
An example of a prior art soft-start switch 1 is illustrated in FIG. 2, where MOSFET 10 serves as a voltage-controlled current device with gate 12 coupled to the output of op-amp 20, drain 16 coupled to the input 30 of the soft-start switch 1, and source 14 coupled to the anode of Schottky diode 40. Input 30 of soft-start switch 1 is coupled to a power supply (not shown) with voltage V.sub.0. The output 50 of soft-start switch 1 provides a voltage V.sub.out to load 55. Load 55 may be an active load. Schottky diode 40 is included to prevent current from being drawn back into soft-start switch 1 if there is a failure in the power supply, but otherwise it is not important to the functioning of the soft-start switch. A reference voltage V.sub.ref, where V.sub.ref &lt;V.sub.0, is provided to terminal 62 of resistor 60 with resistance R. To node 70 is coupled the other terminal of resistor 60, the non-inverting input 22 of op-amp 20, and one terminal of capacitor 90 with capacitance C. The other terminal of capacitor 90 is grounded. Switching means 80 can ground node 70, thereby discharging capacitor 90 and grounding the non-inverting input 22 of op-amp 20. The inverting input 24 of op-amp 20 is coupled to output 50, thus providing feedback by way of the output of op-amp 20 controlling the gate voltage of MOSFET 10, thereby controlling the drain-source current and in turn the voltage V.sub.out applied to load 55. The output voltage of op-amp 20 is assumed to lie between ground and some voltage V.sub.cc, where V.sub.cc is sufficient to put MOSFET 10 into or close to saturation. Without loss of generality we let the ground voltage be zero.
The MOSFET is OFF (V.sub.out =0) when switching means 80 grounds node 70. Assuming capacitor 90 has been fully discharged, soft-start switch 1 initiates a soft-start power-up when switching means 80 decouples node 70 from ground, thereby allowing capacitor 90 to charge. Thus, the voltage of non-inverting input 22 is given by V.sub.ref [1-exp(-t/RC)]. Because of the feedback loop, the op-amp adjusts the gate voltage of MOSFET 10 so that V.sub.out =V.sub.ref [1-exp(-t/RC)], thus providing the soft-start capability with V.sub.out given in FIG. 1.
Switching means 80 may perform a current limiting function by switching MOSFET 10 OFF when too much current is being drawn through the MOSFET and into the load. FIG. 3 illustrates a prior art soft-start switch with current limiting. Components in FIG. 3 are referenced by the same numeral as corresponding identical components in FIG. 2. The soft-start switch of FIG. 3 is a modification of soft-start switch 1 of FIG. 2 in which a sense resistor 100 is placed in the current path from MOSFET 10 to load 55. The voltage drop .DELTA.V across sense resistor 100 is coupled via 102 and 104 to switching means 80. When .DELTA.V is greater than some reference voltage, indicating that the current is too large, switching means 80 grounds node 70, thereby turning the MOSFET OFF.
It should be appreciated that the prior art soft-start switch of FIGS. 2 or 3 regulates V.sub.out in the sense that the drain-source current of MOSFET 10 is controlled via its gate-source voltage so that V.sub.out is made to follow the non-inverting voltage of op-amp 20. However, it may be more desirable to regulate the voltage drop V.sub.0 -V.sub.out rather than the voltage V.sub.out. For example, more than one power supply may provide power to a soft-start switch, where one power supply serves as a back-up for the others. The system may be designed so that one power supply can handle all the power requirements, but it is desirable that all functioning power supplies share equally in supplying power to the load. Unbalanced load sharing may happen when the power supply with the Largest output voltage supplies most of the current, and thereby most of the power to the load. To achieve load sharing, the power supplies are built such that the output voltage of a power supply is gradually lowered when it is determined that there is unequal load sharing. It is therefore desirable that V.sub.out also drop gradually in the same amount that V.sub.0 drops when equal load sharing is sought. Consequently, it is more desirable to regulate the voltage drop V.sub.0 -V.sub.out than V.sub.out.
Another problem associated with the prior art soft-start switch of FIGS. 2 or 3 arises when a capacitive load is hot-plugged to the soft-start switch. For example, a hard-disk when first powered-up presents a capacitive load. It is desirable that a hard-disk drive can be unplugged from the system and replaced with another hard-disk drive "hot-plugged" into the system, i.e., the new hard-disk drive is coupled to a soft-start switch without powering down the system. Hot-plugging a capacitive load brings V.sub.out momentarily close to zero, thereby increasing the voltage drop across the drain and source terminals of MOSFET 10 to approximately V.sub.0. Because of parasitic capacitances between the gate and drain and between the gate and source inherent in a MOSFET, the sudden increase in voltage drop across the drain and source terminals induces a sudden increase in gate-source voltage. Because the MOSFET is a transconductance device (it is a voltage-controlled current source), this increase in gate-source voltage results in an undesirable high source-drain current. Although switching means 80 will eventually turn the MOSFET OFF when a large current surge is detected, it is more desirable that the MOSFET never turn ON in the first place. Therefore, it is advantageous that a soft-start switch with no load connected has the MOSFET turned OFF (gate-source voltage less than the MOSFET threshold voltage) even though switching means 80 is not grounding node 70 and capacitor 90 is charged, and that the switching means keeps the MOSFET OFF even when a capacitive load is hot-plugged to the soft-start switch.
Yet another problem associated with the prior art switch of FIG. 3 is that power is dissipated through the sense resistor 100. Although sense resistors have small resistance, a load may draw several or more amps (for example a hard-disk drive), and therefore the heat dissipation of sense resistor 100 must be accounted for. Also, accurate sense resistors add an additional cost.
Therefore, it is desirable that the prior art soft-start switch of FIGS. 1 or 2 be improved such that the voltage drop V.sub.0 -V.sub.out is regulated, the MOSFET is held OFF when no load is applied or when a capacitive load is hot-plugged, and current limiting is accomplished without a sense resistor. The embodiments of the present invention described hereinafter accomplish these improvements.